Thrombocytopenia is a common complication of HIV-l infection. It is multifactorial in etiology, involving immune-mediated peripheral platelet destruction, as well as impaired thrombocytopoiesis. Recent studies have indicated that megakaryocytes express CD4 on their surface, and are targets for HIV infection. The current study will answer the following questions and will enhance our understanding of this disorder and improve its treatment: AIM 1. Does the virus load in the megakaryocyte or other marrow cells correlate with the development of HIV-1-associated thrombocytopenia? Bone marrow aspirates from HIV-1-infected patients, with or without thrombocytopenia, and from control uninfected patients with autoimmune thrombocytopenia will be analyzed for HIV-1 DNA and RNA load by PCR in situ hybridization and standard PCR analysis. AIM 2. Does virus tropism or cytopathicity for megakaryocytes correlate with the development of HIV-1-associated thrombocytopenia? The bone marrow mononuclear cell samples listed above will be cocultured with megakaryocytic cell lines, primary lymphocytes, or primary macrophages to determine if isolates tropic for, or cytopathic for, megakaryocytes are found specifically in patients with HIV-associated thrombocytopenia. AIM 3. What viral determinants regulate infection and/or cytopathicity of megakaryocytes? The applicants will isolate molecular clones of one or more strains of HIV-1 which are either tropic or cytopathic for megakaryocytes, and identify, by constructing chimeras of proviral clones and non-tropic or non-cytopathic clones, the minimal viral determinants that regulate this property. AIM 4. How do viral factors regulate infection, replication, and cytopathicity in megakaryocytes? The applicants will assess the step in virus replication that is specifically restricted in HIV-1 strains not capable of infecting megakaryocytes, by examining, specifically, virus entry, nuclear import of viral DNA, viral transcription, and virus protein and particle expression. In addition, they will determine if HIV-1 has indirect cytotoxic effects on megakaryocytes, and whether it can be overcome with megakarypoietic interleukins.